Cell Separator for Minimizing Thermal Runaway Propagation within a Battery Pack

ABSTRACT

A spacer assembly for use with a cell mounting bracket in a battery pack is provided. The spacer assembly, comprised of one or more spacers, maintains the positions of the batteries within the battery pack during a thermal event and after the cell mounting bracket loses structural integrity due to the increased temperature associated with the thermal event. By keeping the battery undergoing thermal runaway in its predetermined location within the battery pack, the minimum spacing between cells is maintained, thereby helping to minimize the thermal effects on adjacent cells while ensuring that the cooling system, if employed, is not compromised. As a result, the risk of thermal runaway propagation is reduced.

FIELD OF THE INVENTION

The present invention relates generally to batteries, and moreparticularly, to a means for minimizing the propagation of thermalrunaway within a battery pack.

BACKGROUND OF THE INVENTION

Batteries can be broadly classified into primary and secondarybatteries. Primary batteries, also referred to as disposable batteries,are intended to be used until depleted, after which they are simplyreplaced with one or more new batteries. Secondary batteries, morecommonly referred to as rechargeable batteries, are capable of beingrepeatedly recharged and reused, therefore offering economic,environmental and ease-of-use benefits compared to a disposable battery.

Although rechargeable batteries offer a number of advantages overdisposable batteries, this type of battery is not without its drawbacks.In general, most of the disadvantages associated with rechargeablebatteries are due to the battery chemistries employed, as thesechemistries tend to be less stable than those used in primary cells. Dueto these relatively unstable chemistries, secondary cells often requirespecial handling during fabrication. Additionally, secondary cells suchas lithium-ion cells tend to be more prone to thermal runaway thanprimary cells, thermal runaway occurring when the internal reaction rateincreases to the point that more heat is being generated than can bewithdrawn, leading to a further increase in both reaction rate and heatgeneration. Eventually the amount of generated heat is great enough tolead to the combustion of the battery as well as materials in proximityto the battery. Thermal runaway may be initiated by a short circuitwithin the cell, improper cell use, physical abuse, manufacturingdefects, or exposure of the cell to extreme external temperatures.

Thermal runaway is of major concern since a single incident can lead tosignificant property damage and, in some circumstances, bodily harm orloss of life. When a battery undergoes thermal runaway, it typicallyemits a large quantity of smoke, jets of flaming liquid electrolyte, andsufficient heat to lead to the combustion and destruction of materialsin close proximity to the cell. If the cell undergoing thermal runawayis surrounded by one or more additional cells as is typical in a batterypack, then a single thermal runaway event can quickly lead to thethermal runaway of multiple cells which, in turn, can lead to much moreextensive collateral damage. Regardless of whether a single cell ormultiple cells are undergoing this phenomenon, if the initial fire isnot extinguished immediately, subsequent fires may be caused thatdramatically expand the degree of property damage. For example, thethermal runaway of a battery within an unattended laptop will likelyresult in not only the destruction of the laptop, but also at leastpartial destruction of its surroundings, e.g., home, office, car,laboratory, etc. If the laptop is on-board an aircraft, for examplewithin the cargo hold or a luggage compartment, the ensuing smoke andfire may lead to an emergency landing or, under more dire conditions, acrash landing. Similarly, the thermal runaway of one or more batterieswithin the battery pack of a hybrid or electric vehicle may destroy notonly the car, but may lead to a car wreck if the car is being driven orthe destruction of its surroundings if the car is parked.

One approach to overcoming this problem is by reducing the risk ofthermal runaway. For example, to prevent batteries from being shortedout during storage and/or handling, precautions can be taken to ensurethat batteries are properly stored, e.g., by insulating the batteryterminals and using specifically designed battery storage containers.Another approach to overcoming the thermal runaway problem is to developnew cell chemistries and/or modify existing cell chemistries. Forexample, research is currently underway to develop composite cathodesthat are more tolerant of high charging potentials. Research is alsounderway to develop electrolyte additives that form more stablepassivation layers on the electrodes. Although this research may lead toimproved cell chemistries and cell designs, currently this research isonly expected to reduce, not eliminate, the possibility of thermalrunaway. Accordingly, what is needed is a means for minimizing thermalrunaway propagation, thereby limiting the risks and damage associatedwith such an event. The present invention provides such a means.

SUMMARY OF THE INVENTION

The present invention provides a spacer assembly for use with a cellmounting bracket in a battery pack. The spacer assembly, comprised ofone or more spacers, maintains the positions of the batteries within thebattery pack during a thermal event and after the cell mounting bracketloses structural integrity due to the increased temperature associatedwith the thermal event. By keeping the battery undergoing thermalrunaway in its predetermined location within the battery pack, theminimum spacing between cells is maintained, thereby helping to minimizethe thermal effects on adjacent cells while ensuring that the coolingsystem, if employed, is not compromised. As a result, the risk ofthermal runaway propagation is reduced.

In at least one embodiment of the invention, a thermal runawaypropagation prevention system for use with a plurality of batterieswithin a battery pack is provided, the system comprised of a cellmounting bracket configured to hold the batteries in predeterminedlocations and with a predetermined minimum battery-to-battery separationdistance, and a spacer assembly for maintaining the batteries withintheir predetermined locations and for maintaining the minimumbattery-to-battery separation distance. The material comprising thespacer assembly has a higher melting temperature than that of the cellmounting bracket. Preferably the melting temperature of the spacerassembly is greater than 300° C.; alternately, greater than 500° C.;alternately, greater than 800° C.; alternately, greater than 1000° C.The spacer assembly may be comprised of a material selected from thegroup consisting of aluminum oxide, magnesium oxide, silicon dioxide,silicon nitride, silicon carbide, alumina silicate, aramid paper,silicone coated fiberglass, acrylic coated fiberglass, vermiculitecoated fiberglass, graphite coated fiberglass, polytetrafluoroethylenecoated fiberglass, or some combination thereof. The spacer assembly maybe comprised of at least one strip of material, for example acompressible material, where the spacer assembly is interwoven throughthe batteries. The spacer assembly may be comprised of a plurality ofindependent, rigid spacers configured to fit between adjacent batteries,the spacers being friction fit or bonded into place. The spacers mayhave a height dimension of between 1% and 5% of the overall batteryheight. The spacer assembly may be integrated within the cell mountingbracket, for example by molding the bracket around the spacer assemblyor inserting the spacer assembly into corresponding regions within thebracket. The spacer assembly may be comprised of loose fill spacermaterial that is packed between adjacent batteries.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic components in a conventional battery pack;

FIG. 2 illustrates an alternate configuration of a conventional batterypack in which the cell mounting brackets include a plurality of locatortabs;

FIG. 3 illustrates an embodiment of the invention using spacer strips;

FIG. 4 is a cross-sectional view of the embodiment shown in FIG. 3;

FIG. 5 is a cross-sectional view of an alternate spacer strip design foruse with the embodiment shown in FIG. 3;

FIG. 6 provides a top view of an alternate preferred embodimentutilizing rigid spacers;

FIG. 7 provides a top view of an alternate preferred embodimentutilizing spacers integrated within the cell mounting bracket;

FIG. 8 is a cross-sectional view of a portion of the cell mountingbracket shown in FIG. 7;

FIG. 9 provides a top view of the spacer assembly of FIG. 7 withoutinclusion of the cell mounting bracket;

FIG. 10 provides a top view of a modification of the embodiment shown inFIG. 7 in which the spacer assembly does not utilize cell sleeves;

FIG. 11 is a cross-sectional view of a portion of the cell mountingbracket taken along plane A-A of FIG. 10;

FIG. 12 is a perspective view of one of the spacers shown in FIGS. 10and 11;

FIG. 13 is a top view of a modification of the embodiment shown in FIG.7 in which the spacers are similar to those shown in FIG. 6; and

FIG. 14 provides a cross-sectional view of view of a battery pack usinga loose fill spacer material.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

In the following text, the terms “battery”, “cell”, and “battery cell”may be used interchangeably and may refer to any of a variety ofdifferent cell chemistries and configurations including, but not limitedto, lithium ion (e.g., lithium iron phosphate, lithium cobalt oxide,other lithium metal oxides, etc.), lithium ion polymer, nickel metalhydride, nickel cadmium, nickel hydrogen, nickel zinc, silver zinc, orother battery type/configuration. The term “battery pack” as used hereinrefers to multiple individual batteries contained within a single pieceor multi-piece housing, the individual batteries electricallyinterconnected to achieve the desired voltage and capacity for aparticular application. It should be understood that identical elementsymbols used on multiple figures refer to the same component, orcomponents of equal functionality. Additionally, the accompanyingfigures are only meant to illustrate, not limit, the scope of theinvention and should not be considered to be to scale.

FIG. 1 is a simplified view of a portion of a conventional battery pack100. Although batteries 101A-101E are shown as having a cylindricalform, for example utilizing the 18650 form-factor, it should beunderstood that the problems outlined below as well as the solutionsoffered by the present invention are equally applicable to bothcylindrical batteries and those utilizing a different form-factor, e.g.,pouch cells, rectangular cells, etc. It will also be appreciated thatwhile FIG. 1 only shows a few cells, i.e., cells 101A-101E, a batterypack may include as few as two cells (e.g., flashlights, laptops, etc.),or as many as thousands of cells (e.g., hybrid and electric vehicles).Typically battery pack 100 uses brackets 103/105 to hold the cells inplace, brackets 103/105 constituting the entire battery pack housing, oronly an internal component of the housing. Each bracket 103/105 includesmeans for holding cells 101A-101E within predetermined locations andwith a minimum battery-to-battery spacing. It will be appreciated that aconventional battery pack may use any of a variety of means to hold thecells in place. For example, in the embodiment illustrated in FIG. 1,each bracket 103/105 includes a plurality of indentations or apertures107 that are suitably sized to receive a portion of each cell as shown,indentations/apertures 107 being formed during bracket fabrication viamolding, milling or other well-known processes. FIG. 2 shows analternate configuration in which brackets 103/105 include a plurality oflocator tabs 201, locator tabs 201 being used to hold the cells inplace. As previously noted, battery pack 100 may include a secondary,external housing 109. Secondary housing 109 may be used to providefurther protection to the cells, additional cell isolation, or animproved battery pack mounting structure. Battery pack 100 also includesa connector plate (not shown) or some other means of electricallyconnecting the individual battery interconnects 111.

In a conventional battery pack, the material selected for cell mountingbrackets 103/105 preferably meets several design requirements. First,the selected material should lend itself to easy fabrication techniques,for example injection molding, thus expediting manufacturing whileminimizing cost. Second, the brackets should be relatively low mass, amaterial characteristic that is especially important in large batterypacks where overall mass is critical, e.g., hybrid and electricalvehicles. Third, since the cell casing is typically a cell terminal, thematerial should be electrically non-conductive. Even in those instancesin which the cell casing is not a cell terminal, or is covered by anon-conductive material, an electrically non-conductive cell bracket isstill preferred in order to minimize the risks of battery shorting, forexample during handling. Accordingly, to meet these design requirements,a conventional cell mounting bracket is typically manufactured from apolymer, e.g., a plastic such as nylon.

In a conventional cell, such as a high energy density lithium ion cell,a variety of different abusive operating/charging conditions and/ormanufacturing defects may cause the cell to enter into thermal runaway,where the amount of internally generated heat is greater than that whichcan be effectively withdrawn. During such an event, a large amount ofthermal energy is rapidly released, heating the entire cell up to atemperature of 900° C. or more. Due to the increased temperature of thecell undergoing thermal runaway, the temperature of adjacent cellswithin the battery pack will also increase. If the temperature of theseadjacent cells is allowed to increase unimpeded, they may also enterinto a state of thermal runaway. Accordingly, it is critical thatmeasures are taken to decrease the likelihood of a single thermalrunaway event propagating throughout the battery pack.

One technique that may be used to prevent the thermal runaway of asingle cell from propagating throughout the remaining batteries within abattery pack is to limit the thermal transfer between adjacent cells.Since the thermal runaway of a single cell is a relatively rapid event,and given the inefficient transfer of thermal energy via convectionthrough air relative to conduction, one of the simplest approaches tominimizing thermal runaway propagation is by separating the cells withinthe battery pack. Given the event's short duration, even a smallseparation distance can dramatically improve the resistance to thermalrunaway propagation. Accordingly, it is very important that the cellswithin a battery pack remain in their designated positions during athermal runaway event such that conductive heat transfer between cellsis minimized.

In a conventional battery pack where the cell mounting brackets, e.g.,brackets 103/105, are fabricated from a polymer or similar material, theincreased temperature associated with a thermal runaway event may causethe regions of the brackets in close proximity to the cell undergoingthermal runaway to melt or vaporize. As a result, the cell may no longerbe held rigidly in position, thus allowing the distance between theaffected cell and neighboring cells to differ from the intended spacing.In some applications, for example where the cells are stacked asillustrated in FIGS. 1 and 2, gravitational forces may expedite cellmovement once the bracket(s) begins to melt and/or vaporize. As theaffected cell moves, the spacing between cells may be diminished,leading to decreased resistance to thermal runaway propagation. Cellmovement may also decrease the effectiveness of the battery pack coolingsystem, assuming one is used, thus further lowering the battery pack'sresistance to thermal runaway propagation. Lastly, it should beappreciated that if the affected cell moves sufficiently, it may come torest against an adjacent cell, thereby changing the heat transferprocess from radiation and convection-based, to a combination ofradiation, convection and the more thermally efficient process ofconduction.

In accordance with the invention, one or more spacers are used within abattery pack to reduce the movement of a cell undergoing thermal runawayand ensure that it remains in position. By keeping the affected cell inposition, the minimum spacing between cells is maintained, therebyhelping to minimize the thermal effects on adjacent cells while ensuringthat the cooling system, if employed, is not compromised. As a result,the risk of thermal runaway propagation can be managed.

Regardless of the configuration used for the spacer(s), it is fabricatedfrom a material, preferably a ceramic, that is capable of withstandingthe temperatures associated with thermal runaway without melting,vaporizing or deforming, thus ensuring that the affected cell remains inits intended position for the duration of the thermal event. Exemplaryceramic materials suitable for use with the invention include aluminumoxide (alumina), magnesium oxide (magnesia), silicon dioxide (silica),silicon nitride, silicon carbide (carborundum) and alumina silicate.Other suitable materials include aramid and coated fiberglass, e.g.,fiberglass coated with silicone, acrylic, vermiculite, graphite, orpolytetrafluoroethylene (PTFE). Preferably the spacer has a meltingtemperature of at least 300° C., more preferably of at least 500° C.,still more preferably of at least 800° C., and yet still more preferablyof at least 1000° C. The material used for the spacer(s) depends, inpart, on the design of the spacer and the preferred manufacturingtechniques for the spacer. The material's selection also depends on howthe spacer(s) is intended to interface with the batteries and batterypack components. Preferably the material selected for the spacer(s) hasa relatively low mass, thus minimizing its contribution to overallbattery pack mass. The selected material should also have a relativelylow coefficient of thermal conductivity, thus ensuring that it does noteffectively transfer thermal energy from the affected cell to theneighboring cells. Additionally, the selected material should beelectrically insulating.

FIG. 3 provides a top-view of a portion of a battery pack 300. This viewshows a plurality of cylindrical cells 301 as well as a lower cellmounting bracket 303. A first high temperature spacer strip 305 islocated between the first and second rows of cells 301; a second hightemperature spacer strip 306 is located between the second and thirdrows of cells 301; and a third high temperature spacer strip 307 islocated between the third and fourth rows of cells 301. As shown, strips305-307 are interwoven through the mounted cells. It will be appreciatedthat battery pack 300 may be comprised of either a fewer or a greaternumber of cells, and thus may require a different number of hightemperature spacers. Additionally, a single high temperature spacer maybe continuously interwoven through all of the cells, rather thanbreaking the spacer into regions as illustrated.

In the embodiment illustrated in FIG. 3, preferably the high temperaturespacers, e.g., spacers 305-307, are fabricated from a compressiblematerial. The use of a compressible material ensures that the cellsremain in place, even if the cell mounting brackets, e.g., lower bracket303, melts or vaporizes near the affected cell. Using a compressiblematerial also helps to keep the spacer(s) in position. The degree ofdesired compressibility depends, in part, on the rigidity of the cellmounting brackets since the more inflexible the cells are to movement,the more important it is to use a compressible spacer so that it can beproperly located between the cells. It will be appreciated that thereare a large number of flexible materials that can be used for spacers305-307, such materials exhibiting a high melting temperature, lowthermal conductivity, electrically insulating and sufficientcompressibility. Exemplary materials include alumina-based cloths andfelts, aramid paper (e.g., Nomex® aramid paper), and fiberglass cloththat is preferably coated with silicone, acrylic, vermiculite, graphiteor polytetrafluoroethylene (PTFE).

Since spacers 305-307 are not intended to act as a thermal shieldbetween cells, but are instead intended to keep the cells in placeduring thermal runaway, it is unnecessary for the spacers to run thefull length of the cells, i.e., from lower bracket 303 to upper bracket401 as illustrated in the cross-sectional view of FIG. 4 (taken alongplane A-A of FIG. 3). Accordingly in order to save mass, preferably eachspacer is comprised of a pair of much smaller spacers, i.e., an upperspacer and a lower spacer. For example, in the cross-sectional view ofFIG. 5, spacer 305 is replaced by an upper spacer 505A and a lowerspacer 505B. Similarly, spacer 306 is replaced by an upper spacer 506Aand a lower spacer 506B, and spacer 307 is replaced by an upper spacer507A and a lower spacer 507B. Although a single spacer may be used, forexample one located near the top, bottom, or middle of the cells, theuse of a single spacer is not preferred as it still permits limited cellmovement. For example, if a single spacer is located near the top of thecells, during a thermal runaway event the lower portion of the affectedcell may still move, allowing a portion of the affected cell to contactan adjacent cell, potentially leading to propagation of the thermalevent.

FIG. 6 provides a top-view of a portion of a battery pack 600. As withthe embodiment illustrated in FIG. 3, this view shows a plurality ofcylindrical cells 601 and at least a portion of a lower cell mountingbracket 603. In this embodiment, instead of weaving a compressiblespacer between adjacent cells, independent rigid spacers 605 arepositioned between adjacent cells. It will be appreciated that the exactshape of the spacers in this embodiment will depend upon the shape andmounting configuration for the cells, and that the design shown in FIG.6 is merely illustrative of the approach. Preferably spacers 605 have arelatively low profile, i.e., instead of running the full length of thecells, they only cover a small region of the cell in a manner similar tothat illustrated in FIG. 5 for the compressible spacer. In one exemplaryconfiguration, the height (i.e., profile) of the spacers is only 1-5% ofthe overall height of cells 601. As with the prior embodiment,preferably a pair of rigid spacers 605 is used between adjacent cells,rather than a single low-profile spacer (i.e., an upper spacer and alower spacer as illustrated in FIG. 5 relative to the compressiblespacer).

Spacers 605 may be friction fit in place, or bonded in place using anadhesive or potting compound. Spacers 605 may be inserted between thecells after the cells have been positioned within lower bracket 603, orduring cell mounting. Spacers 605 may be molded, die cut, machined,vacuum-formed, glass-filled, injection molded or otherwise fabricated.Spacers 605 can be formed from a semi-compressible or non-compressiblematerial. As with the prior embodiment, a high melting temperature, lowthermal conductivity, electrically insulative material is required, suchas a ceramic (e.g., alumina ceramic, glass ceramic, silica ceramic,alumina silicate ceramic, etc.).

FIGS. 7-9 illustrate an alternate preferred embodiment in which thespacers are integrated within the cell mounting brackets. In the topview of FIG. 7, a portion of a lower cell mounting bracket 701 is shown.Integral to bracket 701 is a high temperature spacer assembly. In thisembodiment the spacer assembly is comprised of a plurality of batterysleeves 703 that are separated by posts 705. Batteries 707 fit withinsleeves 703. FIG. 8 provides a cross-sectional view of a portion of thisembodiment taken along plane A-A of FIG. 7, while FIG. 9 provides a topview of the spacer assembly without bracket 701 and cells 707, thisfigure showing the same portion of the assembly as previously shown inFIG. 7.

It will be appreciated that the sleeve and post configuration shown inFIGS. 7-9 is but one possible variation of a spacer assembly that isintegrated within a cell mounting bracket. For example, FIGS. 10-12illustrate a variation of this configuration in which the spacerassembly is comprised solely of cell separation posts 1001 that areintegrated within the cell mounting bracket 1003. FIG. 13 illustratesanother variation of this configuration in which a spacer 1301, similarto spacer 605 of FIG. 6, is integrated within cell mounting bracket1303. Note that due to its integration within the cell mounting bracket,spacer 1301 may be designed with less strength and mass than spacer 605while still achieving the goals of the invention.

In general, the spacer assembly selected for integration within the cellmounting bracket depends on the type and shape of the cells employedwithin the battery pack as well as the intended spacer and bracketmanufacturing processes. For example, in one manufacturing process, thespacer assembly is first fabricated and then the bracket is moldedaround the spacer assembly. In an alternate manufacturing process, thebracket is formed, after which the spacer assembly is inserted withinthe bracket either prior to, or during, cell mounting.

In the embodiments illustrated in FIGS. 3-6, the spacer assembly musthave sufficient structural integrity and strength to withstand routinehandling during assembly and use. Accordingly, an advantage ofintegrating the spacer assembly into the cell mounting bracket asillustrated in FIGS. 7-13 is that the spacer assembly, alone, need haveonly minimal structural integrity. As a consequence, the spacer assemblycan be quite small, adding little mass to the mounting bracket and thuslittle additional mass to the battery pack. As with the previousembodiment, the spacers may be molded, machined, stamped, vacuum-formed,or otherwise fabricated from a high melting temperature, low thermalconductivity, electrically insulative material, e.g., a ceramic such asalumina ceramic, glass ceramic, silica ceramic, alumina silicateceramic, etc.

In another embodiment, loose fill spacer material is used to keep thecells in place during thermal runaway, thereby minimizing the risk ofthermal runaway propagation. FIG. 14 is a cross-sectional view of amounting bracket 1401, a plurality of cells 1403, and loose fill spacermaterial 1405 packed between the cells. In order to ensure thatsufficient loose fill material is added to the battery pack to preventmigration of the material from one location to another, therebydecreasing the effectiveness of the spacer material, preferably thebattery pack is completely filled with the loose fill spacer material.Accordingly it is necessary to locate the cells within the pack and addany additional battery pack components such as the cell interconnectassembly and a cell cooling system, if used, prior to filling theenclosure with the loose fill spacer material.

In the embodiment illustrated in FIG. 14, mounting bracket 1401 not onlyprovides the cell mounting structure, but also the battery pack housingwall. It will be appreciated that bracket 1401 may be comprised ofmultiple pieces without departing from the invention, for example usinga cell mounting structure that forms an interior portion of the batterypack housing. The exact configuration depends on the number and type ofcells, the intended application of the battery pack, the means used tointerconnect the cells, the design of any integral cooling system, etc.

Due to the relatively close cell packing used in most battery packs, itis important to force the loose fill spacer material between the cells,thus preventing cell movement during thermal runaway. Accordingly, in apreferred embodiment an injection manifold is used that includes aplurality of injection ports, the injection ports positioned relative tothe inter-cell spacing such that the loose fill material 1405 is forcedbetween all of the cells 1403. Given that loose fill spacer material1405 is forced between the cells, it should be appreciated that not onlywill material 1405 prevent cell movement during thermal runaway, but itwill also help to thermally isolate the affected cell by minimizing thetransfer of thermal energy via convection and radiation.

As with the prior embodiments, loose fill spacer material 1405 may becomprised of any material that has a high enough melting temperature toprevent cell movement during thermal runaway. Additionally, and aspreviously noted, preferably the material is also electricallyinsulating and has a low thermal conductivity. Typical materials usedfor loose fill spacer 1405 include any of a variety of differentceramics, e.g., alumina ceramic fibers, glass ceramic fibers, silicaceramic fibers, alumina silicate ceramic fibers, etc.

Although the cells in the illustrated embodiments have a cylindricalform, e.g., an 18650 form-factor, as previously noted the invention maybe used with other cell designs, shapes and configurations.Additionally, the invention is not limited to a battery pack with aspecific number of cells or a specific cell interconnect arrangement.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention which is set forth in thefollowing claims.

1. A thermal runaway propagation prevention system for use with aplurality of batteries contained within a battery pack, the thermalrunaway propagation prevention system comprising: a cell mountingbracket configured to hold at least a portion of each of said pluralityof batteries within a predetermined location and with a minimumbattery-to-battery separation distance, wherein said cell mountingbracket is fabricated from a first material with a first meltingtemperature; and a spacer assembly, wherein said spacer assemblymaintains each of said plurality of batteries within said predeterminedlocation and with said minimum battery-to-battery separation distance,wherein said spacer assembly is comprised of a second material with asecond melting temperature, and wherein said second melting temperatureis greater than said first melting temperature.
 2. The thermal runawaypropagation prevention system of claim 1, wherein said second meltingtemperature is greater than 300° C.
 3. The thermal runaway propagationprevention system of claim 1, wherein said second melting temperature isgreater than 500° C.
 4. The thermal runaway propagation preventionsystem of claim 1, wherein said second melting temperature is greaterthan 800° C.
 5. The thermal runaway propagation prevention system ofclaim 1, wherein said second melting temperature is greater than 1000°C.
 6. The thermal runaway propagation prevention system of claim 1,wherein said second material is selected from the group consisting ofaluminum oxide, magnesium oxide, silicon dioxide, silicon nitride,silicon carbide, alumina silicate, aramid paper, silicone coatedfiberglass, acrylic coated fiberglass, vermiculite coated fiberglass,graphite coated fiberglass, polytetrafluoroethylene coated fiberglass,or some combination thereof.
 7. The thermal runaway propagationprevention system of claim 1, wherein said spacer assembly is comprisedof at least one strip of said second material.
 8. The thermal runawaypropagation prevention system of claim 7, wherein said second materialcomprising said at least one strip is compressible.
 9. The thermalrunaway propagation prevention system of claim 7, wherein said at leastone strip of said second material is interwoven through said pluralityof batteries.
 10. The thermal runaway propagation prevention system ofclaim 1, wherein said spacer assembly is comprised of a plurality ofstrips of said second material, wherein said second material iscompressible, wherein a pair of said plurality of strips is interwovenbetween pairs of adjacent batteries of said plurality of batteries, andwherein a first strip of said pair is positioned near a top region ofeach of said adjacent batteries and a second strip of said pair ispositioned near a bottom region of each of said adjacent batteries. 11.The thermal runaway propagation prevention system of claim 1, whereinsaid spacer assembly is comprised of a plurality of independent, rigidspacers, said plurality of rigid spacers configured to fit betweenadjacent batteries of said plurality of batteries.
 12. The thermalrunaway propagation prevention system of claim 11, wherein each of saidplurality of rigid spacers has a height dimension between 1 and 5% ofthe overall height of each of said plurality of batteries.
 13. Thethermal runaway propagation prevention system of claim 11, wherein saidplurality of rigid spacers are friction fit between adjacent batteriesof said plurality of batteries.
 14. The thermal runaway propagationprevention system of claim 11, wherein said plurality of rigid spacersare bonded in place.
 15. The thermal runaway propagation preventionsystem of claim 1, wherein said spacer assembly is integrated withinsaid cell mounting bracket.
 16. The thermal runaway propagationprevention system of claim 15, wherein said cell mounting bracket ismolded around said spacer assembly.
 17. The thermal runaway propagationprevention system of claim 15, wherein said spacer assembly is insertedwithin corresponding regions within said cell mounting bracket.
 18. Thethermal runaway propagation prevention system of claim 15, wherein saidspacer assembly is comprised of a plurality of battery sleeves and aplurality of spacer posts.
 19. The thermal runaway propagationprevention system of claim 15, wherein said spacer assembly is comprisedof a plurality of battery spacer posts.
 20. The thermal runawaypropagation prevention system of claim 1, wherein said spacer assemblyis comprised of loose fill spacer material, wherein said loose fillspacer material is packed between adjacent batteries of said pluralityof batteries.